This invention generally relates to the manufacture of composite articles, and more specifically, to curing composite articles in autoclaves.
The large scale manufacture of composite parts, such as fiberglass epoxy and graphite epoxy, is most often conducted in autoclaves, in which the composite articles are heated at elevated pressures to cure the articles. For instance, the composite articles may be heated inside an autoclave to temperatures between 350.degree. and 400.degree. F. at a pressure of 50 psig. In this process, a flexible fibrous material, referred to as the cloth, is applied onto, or layed-up on, a form, referred to as the tool or the work tool, and then a resin is applied onto, or layed-up on, the cloth. This process may be repeated several times to form several layers of the cloth-resin combination on the tool.
After the desired number of layers have been applied to the tool, air may be withdrawn from the interior of the tool to draw the cloth-resin layers tightly onto the tool. The tool is then placed on a platen located within the autoclave, and the composite material is then heated to cure that material. Typically, the manufacturer or the purchaser of the composite parts designates certain requirements, referred to as curing specifications, that must be met during the curing of the composite parts. For example, a curing specification for a composite part may set forth the maximum and minimum rates at which the temperatures of the tool and the composite part may rise during curing. The specification may also require that the temperature of the composite part be within a given range for a specified period of time, for example, 345.degree. to 365.degree. F. for a minimum of 1 hour and a maximum of 2 hours.
There are two well-known types of autoclaves. A first type contains a platen that is not temperature controlled. In an autoclave of this type, a composite part is heated by passing heated air in the autoclave over the part. When a composite part is cured in this type of autoclave, the composite part is first enclosed, or bagged, in a nylon material and insulation is placed locally on the outside of the bagged tool in an attempt to control the temperature rise through the composite part. Temperature uniformity throughout the part is important in order to obtain the desired curing. Determining how a particular composite part should be bagged, insulated, and positioned in the autoclave in order to achieve the desired uniform heating of the part is a tedious and time consuming task that is accomplished by a trial and error process, which must be repeated for each new tool.
In addition, numerous tools for various composite parts can be placed on a platen and simultaneously cured during one production period or cycle. Each time the tools are placed on the platen in a new configuration, the trial and error process, to determine how to achieve the desired uniform heating of the parts, must be repeated. This is because the heated air flow patterns in the autoclave and, therefore, the tool heating rates are greatly affected by the relative sizes and positions of the tools on the platen.
Also, in this type of autoclave, the effective rate at which heat is transferred to a composite part is very low because of the thermal resistance of the nylon bagging, the insulation and the composite material itself. In addition, the thermal mass of the tools, which are often solid steel, can be very large. The combination of the low heat transfer rates and large thermal masses can cause long curing times. Very often these long curing times exceed the specification requirements for the acceptable curing of a particular part.
A second type of autoclave contains a heated platen capable of being temperature controlled, and a composite part is heated in the autoclave by placing the part on the platen and heating the platen. The heated platen conducts heat to the tool at a rate that is much more uniform and rapid than can be achieved in an autoclave where only heated air is used to heat the tool. This allows the composite part to reach the cure temperature more quickly than in an autoclave in which only heated air is used to heat the part, reducing the total amount of time that the composite part must be heated in the autoclave in order to cure.
This is important because many curing specifications limit the total amount of time that a composite part may be at or above a given temperature, and composite parts, after being cured, are sometimes placed in and reheated in the autoclave. This may be done, for example, to help form other composite parts or to join one composite part to another composite part as the latter part is being formed. Reducing the amount of time that a composite part is initially heated in the autoclave to cure the part, increases the amount of time that the composite part may be later heated in the autoclave during these subsequent procedures, commonly referred to as next assembly procedures.
To cure a composite part in an autoclave of this second type, the part is bagged in a nylon material and placed on a platen, but it is usually not necessary to apply any external insulation to the outside of the bagged tool. Numerous tools for various parts can be placed on the heated platen during any production cycle.
In autoclaves of this type, most of the heat transfer is conducted from the platen to the tool through the contact between the platen and the base of the tool. This heat transfer rate is at least an order of magnitude greater than that from the heated air in the autoclave to the composite part, through the nylon bagging covering the tool. Thus, the heating and curing of the composite part is basically independent of the heated air flow patterns in the autoclave, as well as the relative sizes and/or positions of the tools on the platen. The effective heat transfer rate to the base of the tool is very high and uniform, and this uniform, high heat flux is then distributed by conduction through the tool to the composite material.
Because of these high heating rates, the large thermal masses of the tools is no longer a significant issue, and acceptable curing times that meet the applicable specification can usually be achieved. Because of these advantages, the heated platen autoclave is normally the instrument of choice for the large scale manufacture of composite parts.
Two difficulties may be encountered in the use of heated platen autoclaves that can make it difficult to cure composite material parts properly. The first difficulty--a condition referred to as out of plane--is the localized separation of the tool base plate from the platen during the cure cycle. This separation can be the result of distortion of the base plate and/or the platen caused by large and rapid temperature rises of the base plate and the platen during the curing cycle. This localized separation can also result from the fact that the contiguous surfaces of the platen and the base plate may not be perfectly flat or planar, and this may be caused by tolerances allowed in the design and manufacture of the platen and the tool base plate.
In both of these instances, the out-of-plane condition introduces air gaps in those areas where the tool base plate is not in direct contact with the platen. These gaps can be of variable depth--from 1 to 20 thousandths of an inch. The relative out-of-plane condition of the platen and the tool base is also highly dependant on the precise location and orientation of the tool on the platen.
Since air is a good thermal insulator, the local heat transfer rate at an air gap may be only 1/10th of the rate at those areas where the tool base plate is in contact with the platen. The resulting large differences in the heating rates of different areas of the tool causes non-uniform heating of the tool and the composite material. This difficulty is compounded when multiple parts, such as 10 or more, are cured in a single production cycle. This problem often results in an inability to satisfy the curing specification setting forth the required temperature uniformity of the composite part during the curing cycle. The unequal heat transfer rates caused by localized separation of the platen and the tool base also make it more difficult to repeat the same curing procedure over numerous production cycles or runs.
A second difficulty that can arise when using a heated platen autoclave occurs when the composite part being manufactured requires a tool of varying cross section. This can cause the amount of the tool mass directly above each unit area of the tool base plate to vary significantly across the tool, and this can cause the temperatures of the tool and of the composite part to rise non-uniformly during the curing of the composite part. This difficulty also may result in an inability to satisfy the curing specification requirements.